1. Field
One or more embodiments of the present invention relate to an image photographing apparatus and method, and more particularly, to an image photographing apparatus and method where unit sensors of an image sensor are divided into a high-sensitivity sensor and a low-sensitivity sensor and sense an incident optical signal using the high-sensitivity sensor and the low-sensitivity sensor.
2. Description of the Related Art
Apparatuses relying on image capturing devices, such as apparatuses with built-in cameras, including digital cameras and camera phones, have been widely used.
In general, such a camera includes a lens and an image sensor. The lens collects an optical signal reflected from a subject, and the image sensor detects the optical signal collected by the lens and converts the detected optical signal into an electrical image signal. Image sensors are largely classified into camera tubes and solid-state image sensors. Major examples of the solid-state image sensors include charge coupled devices (CCDs) and complementary metal oxide silicons (CMOSes).
In an image sensor, a dynamic range is defined as the range of intensities of an optical signal that can be sensed by the image sensor. The extent of the dynamic range determines whether a strong optical signal or weak optical signal can been sensed. For example, if the image sensor has a narrow dynamic range, it cannot sense the entire intensity of an incident optical signal. As a result, an image that is too bright or too dark is generated.
To ensure proper image detection, the dark and bright portions of an image can be distinguished by adjusting the exposure time of an incident optical signal. However, the adjustment of the exposure time alone is not sufficient to generate an image with an expanded dynamic range, i.e., the adjustment of the exposure time may not by itself compensate for a narrow dynamic range of an image sensor. That is, in order to maintain a signal level and implement an expanded dynamic range, the exposure time may be reduced when a strong optical signal is received and may be increased when a weak optical signal is received. However, when a strong optical signal and a weak optical signal are received simultaneously, the adjustment of the exposure time is not sufficient to maintain the signal level.
Therefore, it is desirable to expand the dynamic range. Accordingly, an image sensor using a self-reset operation has being studied. In such an operation, the image sensor increases the capacity of accumulated electric charges by taking several exposures during a single frame, and thus expands the dynamic range of the image sensor.
Here, a corresponding light reception unit of such an image sensor using this self-reset operation outputs a signal value of a received optical signal. The output signal value decreases as electric charges for the optical signal are accumulated. Thereafter, if the output signal value is less than a preset threshold value, the light reception unit is then reset, and electric charges are accumulated and compared again. This process is thus repeated multiple times within a single frame. When charge accumulation for the entire frame has completed, i.e., the accumulation of light representative of a frame time length is completed, the total quantity of accumulated electric charge can be identified using the frequency of charge accumulation and a final signal value of the output signal. Based on the identified total quantity of accumulated electric charge, the intensity of the incident optical signal can be determined.
Such an image sensor using this self-reset operation includes a comparator in order to compare an output signal value to a preset threshold value and a counter in order to count the frequency of charge accumulations. In addition, the image sensor must include a memory in order to temporarily store the frequency of charge accumulation. That is, the image sensor has to include elements in addition to those of a conventional image sensor. Accordingly, the size of a unit sensor representing a pixel may be increased over conventional sensors. Similarly, the number of output ports attached to the unit sensor may also have to be increased.